110 HEAT OR CALORIC. 



maining constant, the tension of the confined vapor, will still continue 

 unchanged ; because a quantity of it will be condensed, proportional 

 to the diminution of space, so that in fact, the remaining space con- 

 tains the very same quantity of vapor as it did originally. The same 

 law holds good, whether the vapor is pure or mixed with any other 

 gas."* 



(j.) Mode of estimating the force of vapor. This has been al- 

 ready explained under the head of vaporization. Water is introdu- 

 ced into the Torricellian vacuum, and the depression of the mercury 

 measures the force of the vapor. Vapor being produced at every 

 temperature, even below freezing, a table was constructed by Mr. 

 Dalton to express the force through a wide range of temperature. 

 This table, and the results since obtained by Dr. Ure,f may be in- 

 serted at the end of the volume. At the same distance from the 

 boiling point, the force of vapor is the same in all fluids. 



(k.) Effect of vapor upon gases. It enlarges their volume, and 

 that directly, in proportion to the temperature. J 



Gases are freed from their hygrometric moisture either by intense 

 cold, or what is more usual, by exposing them to substances, which 

 powerfully attract moisture ; muriate of lime, which has been ignit- 

 ed, is the substance which is almost always used, and it is very ef- 

 fectual. 



(/.) Hygrometers. These depend, generally, upon a change of 

 dimensions, in consequence of absorbing or giving out moisture. 

 A human hair becomes elongated by imbibing moisture, and returns 

 to its former dimensions, when the moisture is withdrawn ; this change 

 is measured by an instrument, usually furnished with an index, and 

 a graduated arc. Wood, cord, membrane, whalebone, &c. are simi- 

 larly affected. 



Cords are shortened in wet weather ; this appears to be owing to 

 the enlargement of their diameter, at the expense of their length. 

 It is often observed in a common clothes line ; most remarkably 

 at sea, in the great tension of a ship's rigging during a rain storm, 

 and in the relaxation when dry weather returns. 



The amount of vapor in the air, is estimated with considerable accu- 

 racy by covering the bulb of a thermometer with a piece of linen 

 or silk, and exposing it to the air, when the rapidity and extent of 

 the fall of the mercury will indicate the amount of vapor. 



Upon this principle, is constructed a little instrument,^ called the 

 Rosometer. It is a thermometer, || with a ball of black glass, the up- 



* Turner's Chem. p. 56. t Phil. Trans. 1818. 



\ For Mr. Dalton's formula to correct this result, See Turner's Chemistry, first 

 Eng. Ed. pa. 58. 



Invented by Mr. Jones of London, and Mr. Coldstream, of Leith. 

 ]| Filled either with mercury or alcohol. 



